(1) Field of the Invention
The present invention relates in general to an anti-skid braking system for a motor vehicle, and more particularly to a hydraulic pressure control valve assembly for a so-called double piping braking system which functions to control the increase from input to output hydraulic pressure therein.
(2) Description of the Prior Art
A double piping braking system comprising two braking systems is commonly used in motor vehicles due to its higher dependability. In such a system, the following two arrangements are usually used: (1) an arrangement in which the braking system is so constructed that the hydraulic pressure from one of the outlets of a tandem master cylinder is supplied to the right-front wheel brake cylinder and the left-rear wheel brake cylinder and the pressure from the other outlet is supplied to the left-front wheel brake cylinder and the right-rear wheel brake cylinder, and (2) an arrangement in which two brake cylinders are provided on each front wheel and the braking system is so constructed that the hydraulic pressure from one of the outlets of the master cylinder is supplied to one of the respective right and left front wheel brake cylinders and the left-rear brake cylinder, and the pressure from the other outlet is supplied to the other of the respective right and left front wheel brake cylinders and the right-wheel brake cylinder.
The former arrangement is known as X-piping and the latter as J-J piping. In either piping arrangement, it is necessary to provide a hydraulic pressure control valve assembly, such as a limiting valve assembly or a proportioning valve assembly, in the respective hydraulic pressure supply lines to the rear wheel brake cylinders to compensate for the effective weight transfer of the motor vehicle during braking. Therefore, the conventional double piping brake system requires two hydraulic pressure control valve assemblies. This causes disadvantages from the standpoint of mounting space and vehicle assembly.
Acordingly, there has been previously proposed a single valve assembly for the double piping braking system wherein a pair of proportioning valve assemblies performing the respective hydraulic pressure control functions for the rear wheel braking systems are arranged in a common housing parallel to each other, the respective plunger members of the valve assemblies being biased by a common spring acting on a common spring seat.
In such a valve assembly, however, it is necessary to use a stronger, and thus larger, spring as the common spring, since the spring is compelled to bias the paired proportioning valves in a direction to open them for achieving the desired hydraulic pressure control. The use of such a larger spring not only increases the size of the valve assembly itself, but also increases the mechanical force to overcome the spring, which necessitates strength at a portion to which the spring is engaged.
In such a valve assembly, a measure has been employed in which, when one braking system fails to operate, the entire biasing force produced by the common spring is applid to the proportioning valve of the other braking system to increase the critical hydraulic pressure of the proportioning valve of the other braking system for supplying one of the rear wheel brake cylinders with increased hydraulic pressure. However, it has been recognized that even when such a measure is used in the valve assembly, sufficient braking force for effectively braking the motor vehicle is not produced when one braking system fails to operate.
In both proportioning valves, the critical hydraulic pressure determined by the force of the common spring is constant, thereby making the front and rear wheel brake pressure distribution characteristic constant. However, as is known in the art, in order to obtain an ideal distribution characteristic of the front and rear wheel braking force that enables the front and rear wheels to lock simultaneously, the braking force distribution should vary with change of vehicle weight, and the critical hydraulic pressure should increase with an increase of vehicle weight. In this sense, the hydraulic pressure control valve assembly of the above-mentioned type is not suitable for large-sized trucks and the like, the weight of which varies greatly depending on whether they are empty or loaded.
The present invention is thus presented to eliminate the above-mentioned drawbacks encountered in the conventional valve assembly for the double piping braking system.